In use, a storm water chamber is buried beneath the surface of the earth, to collect storm water, such as runoff from parking lots and the like. In a typical stormwater chamber installation, a multiplicity of chambers is laid into cavities in the earth as large array, and then covered over with gravel, stone or soil. See U.S. Pat. Nos. 5,156,488, 5,511,903 and 5,890,838 for examples of chambers. Often the chambers are placed on and buried in gravel; and overlaid with more gravel or soil or a paved surface for motor vehicle traffic or parking. Thus, it is important that they be structurally sound.